The present invention relates generally to remote sensing. Remote sensing can be defined as the science of deriving information about Earth's land and water areas from images acquired at a distance. It usually relies upon measurement of electromagnetic energy reflected by or emitted from the features of interest.
Remote sensing typically uses an electromagnetic sensor such as a camera (often a digital camera) or radar device, an airborne platform carrying the electromagnetic sensor and a position and attitude sensor.
In order to use the imagery collected by the electromagnetic sensor, one must establish the relationship among the camera or sensor used to capture imagery, the imagery itself, and the ground. However, complexities permeate this relationship, including accounting for Earth's generally curved surface and the flatness of the light-collecting surfaces of sensors, film, images and maps; the distortion created by optical systems such as lenses; and the unique position and orientation (and therefore unique perspective) sensors have in a remote-sensing system. Although one of ordinary skill well understands these complexities, for completeness relevant aspects of the relationship among the sensor, the imagery and the ground, including certain commonly used terms—such as interior orientation and exterior orientation—are discussed below.
Interior orientation defines the internal geometry of a camera or sensor as it existed at the time of image capture. Sensor design, lens distortion and the characteristics of the lens, such as focal length, determine the interior orientation. Lens distortion deteriorates the positional accuracy of image points located on the image plane and occurs when light rays passing through the lens are imprecisely bent, thereby changing the relative ray directions such that the light rays intersect the image plane at deviant positions. As a result, objects in the image will appear distorted or closer or further from each other than they really are.
Exterior orientation defines the position and angular orientation (also known as the attitude) of an image. As shown in FIG. 7, the exterior orientation involves three coordinate systems: the ground-space coordinate system (X, Y, and Z), in which all points on the ground are defined; the focal-plane coordinate system (x, y, z), on which the lens of the sensor focuses light reflected by the ground; and the image-space coordinate system, which is the positive image of the image focused by the sensor on the focal-plane coordinate system. The focal-plane and image-space coordinate systems are displaced along the z-axis by the focal length f. As a result, the perspective center O of the sensor is displaced from the center of the image-space coordinate system o by the focal length f.
Xo, Yo, and Zo define the position of the sensor in the ground-space coordinate system at any given time. Xp, Yp and Zp define the position of a ground point P on the ground-space coordinate system and xp, yp define the position in the image-space coordinate system.
Viewing FIG. 7, one of ordinary skill will appreciate that the attitude of the sensor will affect the position of the ground point's representation in the image-space coordinate system. Therefore, the attitude is typically measured while the sensor captures imagery. In FIG. 7 the attitude is illustrated using three angles, omega (ω), phi (φ) and kappa (κ), which define attitude about the x, y and z axes, respectively, of the focal-plane coordinate system. Since the focal-plane coordinate system and the image-space coordinate system are parallel, the three angles also define the orientation of the image-space coordinate system relative to the ground-space coordinate system.
Viewing FIG. 7 also shows that the distance between the sensor and the ground point affects the location of the representation of that ground point in the image-space coordinate system. In other words, terrain variations (otherwise known as topography) affect the image. Terrain variations are not considered part of the exterior orientation of the sensor and are not accounted for merely by knowing the exterior orientation. Rather, terrain variations are accounted for via external data that describes the terrain, such as a digital elevation model, or DEM.
The background principles just covered allow one of ordinary skill to appreciate the inventor's recognition that there remains an unsatisfied desire for a remote sensing apparatus in which a plurality of sensors can be mounted on a platform and the resultant imagery coregistered and stacked. Each of the plurality of sensors may or may not have a unique perspective. If the sensors each has a unique perspective, no two sensors collect image data from the same area on the earth through the same space that is defined between the image sensor and the field-of-view of the image sensor. The apparatus described as the preferred embodiment should, among other things, satisfy the desire to coregister and stack such a plurality of sensors.